Capacitors have been used in an increasingly wide range of applications as electrical storage devices having large capacity, good charge/discharge cycle characteristics, and reduced self-discharge. Examples of those applications include power sources for vehicles such as electric cars and electric two wheelers, and backup power sources for electronic devices. Such electrical storage devices used as the power sources for vehicles are required not only to have a high energy density but also to operate in a wide temperature range.
A capacitor electrode is obtained by coating a current collector with an electrode mixture containing an active material, a binder, and a conductive assistant, and drying the coating.
For example, the electrode may be obtained by coating aluminum foil or a stainless-steel current collector with electrode mixture slurry prepared by dispersing, in a dispersion medium, activated carbon as an active material, polyvinylidene fluoride (PVdF) as a binder, and carbon black as a conductive assistant, and drying the coating (see Patent Documents 1 and 2).
To increase the capacity, a lithium ion capacitor, which is an asymmetric capacitor having a positive electrode integrating the principle of an electric double-layer capacitor and a negative electrode integrating the principle of a lithium ion secondary battery, has been widely studied in recent years.
For a capacitor, may be used an organic solvent such as propylene carbonate, a nonaqueous electrolytic solution dissolving aliphatic ammonium salt as an electrolyte, and an electrolytic solution such as an aqueous solution of sulfuric acid containing a supporting electrolyte.
A binder for an electrode has been required to have a binding capacity, resistance to an electrolytic solution, resistance to temperature, electrochemical stability, low resistance, and other properties. In particular, improvement has been required in terms of a binding capacity which influences the cycle characteristics of the capacitor, low resistance associated with capacitance, and heat resistance which allows the capacitor to operate in harsh environments.
However, if a fluorine-based resin which has been used as a common binder, such as PVdF, is selected, the resin needs to be used in profusion to compensate for its low binding capacity and flexibility. As a result, the amount of an active material decreases. In addition, discharge characteristics of the capacitor will deteriorate due to an increase in resistance, and a lifetime of the electrode will be shortened.
In contrast, a method of using a styrene-butadiene copolymer (SBR) or an acrylic emulsion as a binder to improve the binding capacity has also been proposed. However, use of only such binders may result in low viscosity, and uniform electrode mixture slurry cannot be obtained. Therefore, a dispersion such as carboxymethyl cellulose (CMC) and hydroxypropyl cellulose need to be used as well (see Patent Documents 3, 4, and 5). Further, the resistance of the electrode may increase disadvantageously because the styrene-butadiene copolymer and the acrylic emulsion are insulating rubber-like materials.